ANNAPOLIS -- So many Marylanders are calling the state government toll-free that the Board of Public Works has awarded a new contract that rolls all 180 of the state's independently arranged "800" numbers into one cost-saving master contract.

The new pact could be worth as much as $221,000 a year to MCI Telecommunications Corp., which bid lower than rivals US Sprint and AT&T to provide 800 service. The Department of General Services estimates the state will save about 51 percent of what it had been spending on interstate toll-free calls, and about 37 percent on what it had been spending on the much more frequent calls within Maryland.

Maryland government currently operates approximately 180 separate 800 numbers, the bulk of them in state agencies that handle health problems, promote economic development, or deal with the state's highways or other modes of transportation.

The state provides toll-free calls for such varied activities as vehicle emissions testing, the Maryland Port Administration, Maryland Public Broadcasting, the governor's office, the state Income Tax Division, the Maryland Automobile Insurance Fund, a poison information hot line, the health department's AIDS Administration, and the Homeowners Tax Credit Program, to name a few.

"Because of the bigger demand, and with state buying power consolidated, we'll be cutting the rates in half that some are paying," said Martin W. Walsh Jr., the general services secretary.

In other, unrelated actions the Board of Public Works:

* Officially turned over the state-owned Montebello Hospital to the private, non-profit University of Maryland Medical System, which already runs University Hospital and its renowned Shock Trauma Center.

The transfer, effective July 1, was approved by the General Assembly this winter.

Montebello is a rehabilitation and chronic care facility and "a natural fit" with the University of Maryland's other health care facilities, said Nelson J. Sabatini, the state's secretary of health and mental hygiene.

Under provisions of the transfer, state employees at Montebello must become employees of the non-profit corporation by Jan. 1, 1993, said Dr. Morton I. Rapoport president and chief executive officer.

* Awarded a three-year contract worth up to $13.8 million to Andersen Consulting of Washington, to provide the software, manpower and training to consolidate all of the state's tax collection systems into one integrated, computerized system.

Chief Deputy Comptroller J. Basil Wisner said Andersen has set up similar consolidated tax systems in New Jersey, Maine, Massachusetts and New York City. The consolidation will be done on an IBM mainframe computer already in use in the Annapolis Data Center.

Once it's fully implemented, Mr. Wisner said tax collection, auditing, cross-referencing of tax information, and other functions should be simplified, eliminating duplication and reducing manpower requirements.